Optical fibers are used extensively in telecommunications systems. It is often necessary to add or drop a wavelength channel in optical links or systems. Channel dropping can be achieved by a re-configurable channel dropping de-multiplexer. Such a de-multiplexer receives, from an input port, a wavelength division multiplexed composite optical signal that comprises several channels, or “wavelengths”. These plural channels which include one or more “dropped channels” or wavelengths that a user wishes to remove, or “drop” to selected local channel drop ports, and other “express” channels that the user wishes to send, as a wavelength division multiplexed composite optical signal, to another destination.
FIG. 7 presents a known architecture re-configurable channel dropping system. A composite optical signal entering the re-configurable channel dropping system 700 from an input fiber optic line 701 is completely separated into its component channels λ1, λ2, . . . , λn, by de-multiplexer 702. Assuming that the input composite optical signal comprises n channels, then, typically, the de-multiplexer 702 comprises a series of n optically coupled channel band pass filter assemblies, 704.1-704.n, wherein each channel band pass filter assembly transmits a different respective one of the channels and reflects other channels. By this method, the channels λ1, λ2, . . . , λn are completely separated and each such separated channel is directed to a respective one of the optical switches 706.1, 706.2, . . . , 706.n. Each of the switches 706.1-706.n can direct a channel either to one of the optical drop lines 708 or to one of the optical lines 710 leading to the multiplexer 703. The multiplexer 703 works oppositely to the de-multiplexer 702 and comprises a second series of channel band pass filter assemblies 705.1-705.n to receive the various express channels from the optical lines 710 and recombine these channels into a composite optical signal delivered to the output 714.
The known re-configurable channel dropping system 700 is re-configurable in the sense that the wavelengths that may dropped may be chosen at the discretion of the user. Although the conventional re-configurable channel dropping system 700 performs its intended function adequately, it requires one switch for each wavelength as well as two separate series of channel band pass filter assemblies. This type of configuration leads to large optical insertion losses, especially for the express channels, since each such express channel must pass through two channel band pass filter assemblies as well as an optical switch. Accordingly, there is a need in the art for an improved re-configurable channel dropping de-multiplexer system that does not require any of the express channels to be transmitted through a channel band pass filter. The present invention addresses such a need.